Method of treating pulp



Patented Feb. 1 1, 1936 UNITED STATES METHOD OF TREATING PULP Arno J. LuthQPort Edwards, Richard A. Nugent, Nekoosa, and Neil H. Christian, Port Edwards,.

Wis.,

assignors to Nekoosa-Edwards Paper Company, Port Edwards, Wis., a corporation of Wisconsin No Drawing. Application October-18,193LI Serial N0. 748,921

11 Claims.

This invention relates to a; method of treating pulp and more particularly to a method of obtaining a bleached pulp having improved char-' acteristics from pulp prepared by the sulphate process, or some similar alkaline process using a sulfide as an active cooking ingredient. The invention also relates to the method of removing phlobotannin bodies from sulphate pulp to improve the bleachability of the pulp. a

The coloring matter in sulphate pulp is due largely to a substance known at phlobotannin, and to derivatives of phlobotannin containing sulphur in the molecule. These phlobotannin bodies are acidic in nature and change color in solutions of varying degrees of alkalinity and acidity. In an alkaline solution, pH 10.5 or above, the oxidized phlobotannin bodies are a dark red brown color and very soluble. In acid solution, pH 2.8 or below, the oxidized phlobotannin bodies are alight brownish yellow in color, but are comparatively insoluble. The phlobotannin bodies are very easily oxidized to form black color bodies, andthey are precipitated by the addition of acids or by alkaline earth alkalies. The oxidized phlobotannin bodies are rendered very soluble in strong alkalies, by chlorination. Before, oxidation the phlobotannin bodies are a bright yellow color in alkaline solution.

In order to improve the bleachability of sulphate pulp, it is necessary to remove these phlobotannin bodies from the pulp prior to bleaching, or at least to materially reduce the quantity of phlobotannin bodies present in the pulp. The phlobotanninbodies require from three to ten times their weight in bleaching powder to change their color from-a brown to a light cream. It is self-evident, therefore, that the elimination of these bodies without the use of drastic bleaching reactions aids very materially in retaining the original fibre strength.

Where the bleached pulp is to be used :in the manufacture of high grade bond, ledger and writing papers, it is highly desirable to ,accomplish the removal of the phlobotannin bodies in sucha way as to obtain a bleached sulphate pulp of exceptionally high purity, that is, a pulp very high in alpha cellulose andhaving a low copper number. It is also desirable in' making high grade papers such as indicated to use a sulphate pulp that has special paper making characteristics, such as resistance to mechanical action in the heaters and Jordan so as to be capable of withstanding a longer beating time and thus of acquiring a higher degree of hydration. A pulp having these improved paper making characteristics can be manufactured into high grade papers having surface characteristics similar to those of rag papers. h

It is therefore an objectof this invention to known.

provide a method ofobtaining bleached sulphatepulp of improved characteristics bythe more complete removal of phlobotanninbodies from the pulp prior-to the bleaching-operations.

It is a further important object of this invention to provide a method of decreasing the bleach demand of sulphate pulp and improving the paper making characteristics of the pulp after bleach-- ing.

It .is a further important object; of this inven tion to provide a method for obtaining a bleached sulphate pulp having a high alpha cellulose content and a low copper number and capable of being manufactured into high grade papers having improved surface characteristics.

Other and further important objects of-this invention will become apparent from the following description and appended claims.

According to our. present invention, sulphate pulp is treated shortly after ithas been blown from the digesters and-before it hashad an opportunity to. come into contact with oxidizing influences, with a'caustic solution to remove phlobotannin bodies. According to the usual custom of manufacturing pulp by the sulphate.

process, at the conclusion of the'cooking period in the digester, the pulp is blown into a diffuser, in which the pulp is washed to remove the cooking liquor, or black liquor as it is commonly In the method of our invention, an aqueous solution of caustic soda is introduced into the top of the diffuserin the space abovethe level of the pulpand is gradually forced downwardly through the mass ofpulp, displacing the black liquor ahead of it. The body. of caustic solution is, in turn, displaced by'a following body of water introduced into the difiuser underpressure. The addition-of the fresh caustic soda solution with the following wash watenproduces a diffusion of maintained-alkalinity capable of absorbing, or maintaining in solution, the phlobotannin bodies and other decomposition products remaining in the pulp from the digestion, which products would ordinarily be oxidized and precipitated on. the pulp by regular washing procedures.

The caustic sodasolution used in treating the sulphate pulp in the difiuser is preferably one that has been preparedby, dissolving caustic soda in fresh water, although so called white liquor,, 1

as it is known in the'sulphate process, which, is a strongly alkaline liquor produced by causticizing green liquor, may be used. The amount of caustic solution used in our method will vary considerably depending-upon the time of contact of the caustic liquor with the pulp, the concentration of the caustic liquor or solution and the temperature of the mass in the diffuser. Ordinarily, however, between, 2.5 and 15% of caustic soda (NaOI-I)" by weight of the air dry pulp undergoing treatment constitutes the most satisfactory operating range. A caustic solution of a concentration of approximately 50% of NaOH by weight of the solution is very satisfactory. Using this percentage of caustic soda and a solution of this concentration, the operation can be efficiently carried out in a period of from 12 to 16 hours in diffusers of theusual size.

The treatment of the pulp in the diffuser is started as soon as possible after the pulp has been blown into the diffuser, so as to avoid any substantial oxidation of the phlobotannin bodies.

A caustic solution of the concentration above indicated is introduced into the space above the pulp in the diffuser and gradually forced downwardly through the mass of pulp and cooking liquor, displacing the cooking liquor ahead of it. After all of the caustic solution has been introduced into the diffuser, it is followed by weak liquor from the process or by wash water, introduced under sufficient pressure, say about 15 pounds per sq. in., to displace the body of caustic solutionahead of it at the desired rate to complete the operation in from 12 to 16 hours. The action of the caustic soda solution is to dissolve the phlobotannin bodies and to remove these bodies from the pulp. g

After the pulp in the diffuser has been thus treated and then washed thoroughly, it is subjected to the usual operation of being passed through a knotter and through screens to prepare the pulp for bleaching.

The bleaching process is preferably of a multistage type, including at least one treatment with gaseous chlorine at comparatively-low density and a subsequent, and preferably final treatment also at low density, with bleaching solutions containing calcium hypochlorite and of maintained caustic alkalinity. Multi-stage bleaching oper ations of this type are not in themselves new, but the method of our invention includes an intermediate treatment of the pulp with a caustic alkali to aid in the removal of color bodies. This treatment with caustic alkali between the chlorinating and calcium hypochlorite bleaching process supplements the treatment of the pulp in the difiuser with caustic soda already described and is apparently necessary to obtain the best results fromthe diffuser treatment.

The first chlorination step may be'carried out with the use of gaseous chlorine and at a consistency of between 3 and 5%. Chlorine is introduced into the aqueous suspension of pulp in any suitable manner, as by bubbling chlorine gas into a downwardly flowing column of pulp at a rate such that the bubbles of chlorine gas do not rise in the column before being absorbed. Suflicient chlorine is introduced in this manner to equal about 45% of the chlorine required for complete bleaching of the pulp. The temperature of the pulp mass during chlorination is preferably maintained at between and F.

After the preliminary bleaching step just described, the pulp is washed and from 0.5, to 3% of caustic soda, basedbn the air dry'weight of the pulp, is added to the washed pulp. The caustic soda may suitably be in a 40 to 50% aqueous so-.

lutlon and ispreferably added continuously to the stock while it is being moved along in the system. After addition of the caustic solution, the stock is passed through a reacting tower, or vessel, wherein it is allowed to remain for 10 to 20 minutes to permit the reaction between caustic soda and any chlorinated bodies, such as lignin and ligno celluloses, to take place. Chlorination of the phlobotannins renders them soluble in strong alkalies, such as caustic soda, after they have been oxidized, whereas they are rendered insoluble and precipitated by alkalies of the alkaline earth metals.

The stock is thereafter again washed and concentrated to 12 to 20% consistency, as by means of a screw press. To the concentrated mass of stock there is added a furth quantity of caustic soda solution in an amount equivalent to about 5% of caustic soda (NaOH) on the air dry weight of the pulp present. The thus treated stock is heated'to a temperature of between and F., preferably about 100 F., and maintained at that temperature for approximately one hour During this treatment, the strong caustic solution dissolves any remaining impurities, such as lignin and phlobotannin bodies that have been chlorinated during the bleaching operations.

After this treatment with caustic soda, the pulp is again washed and then bleached at low density, say from 3 to 6%, with an aqueous bleaching solution of calcium hypochlorite. The amount of bleaching solution added is that calculated to complete the bleaching of the pulp. Bleaching is carried out at a temperature of between 100 and 120 F., preferably at about 115-F. During this final bleaching step, the stock is maintained alkaline to phenolphthalein by the addition of caustic soda if necessary, in accordance with the teaching in the Wells Patent No. 1,883,193, dated October 18, 1932.

When the stock has been completely bleached, it is removed from the bleacher and washed. During washing a dilute mineral acid, such as sulphuric acid, may be added .as a neutralizing agent to remove the last traces of alkali in the stock.

Bleached sulphate pulp made in accordance with the method above described is of higher purity, that is, has a higher alpha cellulose content, than pulp made by a process otherwise similar but omitting the treatment with caustic soda solution in the diffuser and at a point intermediate and chlorinating and bleaching steps. Although some of the advantages of this invention can be secured by treatment of the pulp inthe diffuser with caustic soda, without the subsequent caustic soda treatment during the chlorination step, better results are obtained by the combination of these two steps. The caustic soda treatment of the pulp in a diffuser has been found to reduce the amount of chlorine required for bleaching fromiabout 35% to about 25%, based upon bleaching powder of 35% available chlorine content. v

By carrying out the caustic treatment of the pulp in the diffuser it is possible to recover the chemical used without additional apparatus along with the black liquor by processing the whole in the usual recovery system.

Ordinarily bleached sulphate pulp has the following characteristic pulp tests- Percent mullen after 30 min. in laboratory beater Percent tear after 30 min. in laboratory beater 110 The bleached sulphate pulp produced by our proposed invention has the following character istic pulp tests- Bleached sulphate puip produced in accordance with this invention has improved paper making characteristics in that it resists mechanical action in the beaters and Jordan and therefore may be beaten for a much longer period than many other grades of bleached sulphate pulp. As a result of the ability of the bleached sulphate pulp of our invention to withstand beating, it may be more highly hydrated without destroying the strength of the pulp. Accordingly, the pulp of our invention can be used satisfactorily in the manufacture of high grade bond, ledger and writing papers to produce paper having surface characteristics similar to those of rag bond paper. y

We are aware that numerous details of the process may be varied through a wide range without departing from the principles of this pheric oxygen with a caustic alkali solution to dissolve phlobotannin color bodies, subjecting the treated pulp after washing to a multi-stage bleaching operation and treating said pulp intermediate successive bleaching steps with an alkaline solution to remove color bodies then present.

2. The method of obtainingbleachedfpulp offif' improved characteristics from pulp" preparedby" the use of an alkaline cooking' li'q'uor containing an active sulfide ingredient, which comprises treatingthepulp in a closed vessel under pressure'before being exposed to the oxidizing infiuences of dissolved and atmospheric oxygen with a fresh caustic alkalisolution, to dissolve phlobotannin color bodies, subjecting the treated pulp after washing to a multi-stage bleaching dissolved and atmospheric oxygen with a fresh caustic alkali solution in an amount equivalent to about 2.5 to 15% of NaOH on the air dry weight of the pulp to dissolve phlobotannin color bodies, subjecting the treated pulp after washing to a multi-stage bleaching operation and treating said pulp intermediate successive bleaching steps with an alkaline solution to remove color bodies then present.

I'he method of obtaining bleached sulphate pulp of improved characteristics, which comprises forcing afresh solution of caustic alkali through an enclosed freshly cooked and unwashed mass of sulphate pulp in a heated condition before the pulp has beenexposed to the oxidizing influences of dissolved and atmospheric oxygen to dissolve phlobotannin color bodies therefrom, washing the treated pulp, bleaching the pulp in a plurality of stages, and subjecting said pulp between successive stages to treatment with an aqueous solution of caustic alkali while maintaining the pulp at a temperature above F.

5. The method of obtaining bleached sulphate pulp of improved characteristics, which comprises forcing a fresh solution of caustic alkali through an enclosed mass of sulphate pulp in a heated condition before the pulp has been exposed to the oxidizing influences of dissolved and atmospheric oxygen to dissolve phlobotannin color bodies therefrom, washing the treated pulp, bleaching the pulp in a pluralityof stages, and subjecting said pulp between successive stages to treatment with an aqueous solution of caustic alkali while maintaining the pulp at a consistency of about 18% on an air dry fiber basis and at a temperature above 100 F.

6. The method of obtaining bleached sulphate pulp of improved characteristics, which comprises forcing through an enclosed heated mass of pulp that has only recently been blown from a digester before the pulp is exposed to the oxidizing influences of dissolved and atmospheric oxygen, a fresh solution of caustic soda containing an amount of caustic soda equivalent to-about 2.5 to 15% of the air dry weight of the pulp, washing the treated pulp, bleaching the pulp in a plurality of stages including one stage with gaseous chlorine and a subsequent stage with calcium hypochlorite and heating the pulp intermediate said stages in the presence of about 5% of caustic soda on the air dryweight of pulp.

7. The method of obtaining bleached sulphate pulp of improved characteristics, which comprises forcing through anenclosed heated mass of pulp that has only recently been blown from a digeste'r before -thepulp is exposed to the oxidizing in- '-fiue'nces of dissolved and atmospheric oxygen, a fresh solution of caustic soda containing an amount of caustic soda equivalent to about 2.5 to 15% of the air dry weight of the pulp, washing the treated pulp,'bleaching the pulp in a plurality of stages includingone stage with gaseous chlorine and a subsequent stage with calcium hypochlorite and heating the pulp to a temperature-between 100 and F. intermediate said stages and maintaining the pulp at that temperature for'about an hour inthe presence of about 5% of caustic soda on the air dry weight of pulp.

8. In the process of obtaining a bleached sulphate pulp of improved characteristics, the step of dissolving phlobotannin color bodies from freshly blown pulp with caustic soda in aqueous solution before the pulp is exposed to the oxidizing influences of dissolved and atmospheric oxygen.

9. In the process of obtaining a bleached sulphate pulp of improved characteristics, the step of dissolving phlobotannin color bodies fromfreshly blown pulp with caustic soda in aqueous solution in an amount equivalent to 2.5 to 15% of NaOH on the air dry weight of the pulp before the pulp is exposed to the oxidizing influences of dissolved and atmospheric oxygen.

10. In the process of obtaining a. bleached sulphate pulp of improved characteristics, the steps or adding an aqueous solution of caustic soda on top of a hot, freshly blown mass of sulphate pulp, displacing said solution gradually downwardly through said mass by a following mass of water, and washing said pulp before the pulp is exposed to the oxidizing influences of dissolved and atmospherlc oxygen.

11. In the process of obtaining a bleached sul- 

